1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies and, more particularly, is concerned with a boiling water nuclear reactor (BWR) fuel assembly containing nuclear fuel in a hybrid design for elimination of pellet cladding interaction (PCI) constraints and minimization of PCI failures.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In a nuclear reactor generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator.
In a typical boiling water reactor (BWR) fuel assembly, a bundle of fuel rods in a N by N array are subdivided into four separate mini-bundles by a central water cross and each mini-bundle is supported in laterally spaced-apart relation by a plurality of spacers axially spaced apart along its fuel rods. Then, all four mini-bundles of the fuel assembly are encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. Such BWR fuel assembly is illustrated and described in U.S. Pat. No. 4,560,532 to Barry et al.
Operation of reactors at high power density across the core is desired from the standpoint of operating efficiency. There are, however, practical limits on the power density which can be maintained. These limits are essentially two: namely, fuel and reactor structural material temperature limits, and coolant fluid temperature and pressure limits. One consequence of operating the reactor core so as to approach any of these limits in any region of the core is that the problem of fuel pellet clad interaction (PCI) can arise resulting in possible failure of the clad and chemical reaction of the clad or the fuel with the coolant. This and related problems, as well as some solutions to these problems proposed heretofore, are discussed in detail in U.S. Pat. No. 3,147,191 to Crowther.
The need for avoiding PCI related fuel failures has recently prompted BWR fuel manufacturers to increase the number of fuel rods in a fuel assembly, thereby decreasing the power generated per rod. However, in the case of the BWR fuel assembly described above, increasing the 4.times.4 fuel rod array in each fuel mini-bundle to a 5.times.5 array leads to a drastic reduction in the bundle inlet orificing for hydraulic compatibility with existing BWR reload fuel. This leads to unaccepted degradation in the stability performance of the 5.times.5 array. It arises for the resulting fuel due to the presence of increased wetted area from the water cross in comparison to other BWR fuel assemblies having conventional open lattice fuel designs. The wetted area from the water cross refers to the additional flow frictional area due to the cold walls within the fuel assembly.
Consequently, a need exists for a technique to improve BWR fuel design so as to avoid the potential PCI constraints and failures, but without, at the same time, producing other unacceptable side effects.